SBS can impose a practical upper limit on the amount of useful power that can be transmitted by an optical waveguide, such as an optical fiber. Without wishing to be bound by any particular theory, SBS is understood to be a non-linear acousto-optic effect wherein responsive to electromagnetic energy propagating in the forward direction, electromagnetic energy is scattered in the backward direction accompanied by the release of phonon energy. The backward propagating electromagnetic energy is down shifted in frequency from the forward propagating electromagnetic energy.
SBS leads to a drop in transmission efficiency above a threshold input power, as is well understood in the art. Once the threshold input power has been reached, launching additional power in the forward direction results in little or no increase in power being transmitted the forward direction; most or all of the additional input power is scattered in the backward direction. The SBS threshold power can depend on the fiber length and the power density. The build-up of SBS is greater at longer lengths. Similarly, a smaller core size increases the power density and hence quickens the onset of SBS. SBS can limit one or both of the amount of power that can be transmitted as well as the distance over which a given amount of power can be transmitted. SBS is particularly severe for the transmission of electromagnetic energy having a narrow line width, such as is often provided by laser sources. For example, major industrial laser systems currently transmit 4–5 kW of Nd-YAG power over about a 50 meter length of fiber. It can be difficult to transmit such power levels over more than 50 meters. One of the factors limiting the length is SBS.
The limitations that SBS can impose on the transmission of electromagnetic energy over optical fibers is well known, and various forms or constructions of fiber have been proposed in the past with the aim of reducing or suppressing the SBS phenomenon. Examples of such fiber constructions are disclosed in U.S. Pat. Nos. 5,170,457; 5,848,215, 6,542,683 B1; 6,687,440 B2 and 6,587,623 B1; in U.S. Patent Application Publication Nos. 2001/0019642 A1 and 2004/0037529 A1; and in Japanese Patent Numbers 1004709; 1129207 and 10096828.
While some of the foregoing applications or patents may describe fibers that represent an improvement in the art, each can have drawbacks or limitations in certain circumstances. Accordingly, it is an object of the present invention to provide methods and apparatus that address one or more of the drawbacks or deficiencies of the prior art. Other objects will be apparent below, or from practice of the invention as taught herein.